Surface coating for degradable magnesium and magnesium alloys and method for preparing the same

ABSTRACT

A surface coating for degradable magnesium and magnesium alloys, including: an inner layer and an outer layer. The inner layer is a magnesium phosphate conversion layer, and the outer layer is a hydroxyapatite layer. A method for fabricating the surface coating is also provided herein, which includes: soaking the magnesium or magnesium alloy in an acidic solution containing magnesium salt and phosphate under heating to form the magnesium phosphate conversion layer on a surface of the magnesium or magnesium alloy; and transferring the magnesium or magnesium alloy to an alkaline solution containing calcium salt and phosphate followed by soaking under heating to form a hydroxyapatite layer on a surface of the magnesium phosphate conversion layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Application No. PCT/CN2022/115081, filed on Aug. 26, 2022, which claims the benefit of priority from Chinese Patent Application No. 202111370263.8, filed on Nov. 18, 2021. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This application relates to surface coatings for magnesium and magnesium alloys, and more particularly to a surface coating for degradable magnesium and magnesium alloys and a method for preparing the same.

BACKGROUND

At present, the commonly-used clinical bone implant materials mainly include non-degradable stainless steel, titanium alloy and cobalt-based alloy materials, which will remain in the body for a long time after implantation, and affect the surrounding tissues. Moreover, a second surgery is generally needed to remove the metal implant from the patients, which will aggravate the pain and economic burden. Additionally, the above-mentioned materials have a higher elastic modulus than bone tissues, leading to a “stress shielding” effect and affecting the bone tissue repair. Compared with the traditional implant materials mentioned above for bone repair, magnesium alloys have the following advantages: (1) degradability; magnesium alloys have a low corrosion potential, and are prone to corrosion in the presence of chloride ions, such that they can be completely degraded in vivo by corrosion; (2) high bio-safety; as an essential nutrient element, magnesium participates in almost all energy metabolism pathways in the body, and plays an important role in regulating neuromotor function and physiological function, and preventing circulatory system diseases and ischemic heart disease; excess magnesium will be metabolized and decomposed by the urinary system and then excreted; (3) desirable biomechanical compatibility; among all metal materials, magnesium is closest to natural bone in terms of biomechanical properties, which can effectively alleviate the “stress shielding” effect resulted from the traditional medical metal materials.

However, there are still some limitations in the clinical application of magnesium and magnesium alloys as orthopedic implant materials. Magnesium and magnesium alloys are chemically active, and have a large degradation rate after implantation, such that they fail to maintain sufficient structural integrity and mechanical strength before tissue repair and regeneration. Moreover, the degradation is often accompanied by the generation of hydrogen. The excessively rapid degradation will result in the accumulation of hydrogen, affecting the adhesion and proliferation of surrounding osteoblasts and the repair of bone tissues.

Surface coating modification is an effective technical tool to control the degradation of magnesium alloys, and a wide variety of coatings (e.g., polymer coating, magnesium oxide coating and calcium phosphate (Ca—P) coating (e.g., hydroxyapatite coating)) have been currently reported. The Ca—P coatings are similar to the bone tissue in composition, and thus can improve the corrosion resistance and biocompatibility of bone implants. Among the Ca—P coatings, hydroxyapatite has good crystallinity, and is most similar to the bone tissue in composition, which can improve osseointegration ability and accelerate bone tissue healing. In addition, hydroxyapatite has the lowest solubility among the Ca—P coatings, and its degradation products have a slight alkalinity, which is conducive to the further inhibition of degradation of magnesium substrate from the perspective of chemical corrosion equilibrium. Currently, chemical deposition, electrodeposition and sol-gel method are commonly used to prepare the hydroxyapatite coating on the surface of magnesium alloy. However, most of them have defects of complicated preparation, low thickness, loose coating structure and insufficient bonding strength.

Chinese Patent No. 106544714B discloses a method for fabricating the surface coating on Mg—Zn—Ca alloy bone screw, in which raw materials are treated by micro-arc oxidation and electrophoresis, and then subjected to hydrothermal synthesis to prepare a hydroxyapatite coating. However, there are a large number of micro-holes and cracks on the surface of the coating fabricated by micro-arc oxidation and electrophoresis, which provide ion channels for the penetration of corrosive medium, so as to affect the protective performance of the coating against the corrosion and degradation of the magnesium substrate. Chinese Patent No. 103484845B discloses a Ca—P composite coating on ZK60 magnesium alloy, which is fabricated by one-step hydrothermal synthesis. This method still suffers the following deficiencies: (1) this method employs calcium nitrate as calcium source, but does not introduce a complexing agent in the reaction system, which makes it difficult to adjust the pH; (2) the hydrothermal temperature (100-200° C.) is too high, and will affect the mechanical properties of the metal substrate; (3) since it is difficult to directly deposit hydroxyapatite on the surface of magnesium and magnesium alloys, the thickness, coverage and interfacial bonding strength of the coating will be significantly deficient; and (4) composite morphologies with highly bioactive micro- and nano-scale hierarchical structures cannot be formed on the coating surface. Chinese Patent Application Publication No. 111973812A discloses a preparation method for a double-layer coating on magnesium alloys, including a magnesium fluoride inner layer and a hydroxyapatite outer layer. However, the fluoride ions generated from the degradation of the magnesium fluoride layer may have a negative impact on the local bone tissue, and cause fluorosis. Moreover, this preparation process is cumbersome and time-consuming (including the preparation of calcium hydrogen phosphate dihydrate and conversion into hydroxyapatite), has high safety risk due to the use of toxic reagents such as high-concentration hydrogen fluoride.

SUMMARY

An objective of this application is to provide a surface coating for degradable magnesium and magnesium alloys and a method for preparing the same.

Technical solutions of this application are described as follows.

In a first aspect, this application provides a surface coating for degradable magnesium and magnesium alloys, comprising:

an inner layer; and

an outer layer;

wherein the inner layer is a magnesium phosphate conversion layer, and the outer layer is a hydroxyapatite layer.

In an embodiment, the magnesium phosphate conversion layer has a thickness of 50 nm-10 μm; and the hydroxyapatite layer has a thickness of 0.1 μm-500 μm.

In an embodiment, a bonding strength between the surface coating and a magnesium substrate or a magnesium alloy substrate is equal to or larger than 50 MPa; and an atomic ratio of calcium (Ca) to phosphorus (P) in the hydroxyapatite layer is (1.5-1.67):1.

In a second aspect, this application provides a method for preparing the surface coating mentioned above, comprising:

(S1) soaking a magnesium or magnesium alloy substrate in an acidic solution containing a magnesium salt and a first phosphate under heating to form a magnesium phosphate conversion layer on a surface of the magnesium or magnesium alloy substrate; and

(S2) transferring the magnesium or magnesium alloy substrate with the magnesium phosphate conversion layer to an alkaline solution containing a calcium salt and a second phosphate followed by soaking under heating to form a hydroxyapatite layer on a surface of the magnesium phosphate conversion layer.

In an embodiment, in step (S1), the magnesium salt is selected from the group consisting of magnesium nitrate, magnesium chloride, magnesium sulfate, magnesium phosphate, magnesium perchlorate, magnesium acetate, magnesium hydroxide and a combination thereof; a concentration of the magnesium salt in the acidic solution is 0.001˜10 mol/L; the first phosphate is selected from the group consisting of sodium phosphate, potassium phosphate, magnesium phosphate, ammonium phosphate, calcium phosphate, and a combination thereof; and a concentration of the first phosphate in the acidic solution is 0.001˜10 mol/L.

In an embodiment, a molar ratio of magnesium ions to phosphate ions in the acidic solution in step (S1) is (0.2˜5):1, preferably (0.8˜1.2):1.

In an embodiment, in step (S1), the acidic solution is adjusted to pH 2-7 with nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid or a combination thereof; and the soaking is performed at 5˜99° C. for 5 min˜12 h.

In an embodiment, in step (S2), the second phosphate is selected from the group consisting of sodium phosphate, potassium phosphate, magnesium phosphate, ammonium phosphate, calcium phosphate, and a combination thereof; and a concentration of the second phosphate in the alkaline solution is 0.001˜10 mol/L; the calcium salt is selected from the group consisting of calcium phosphate, ethylenediaminetetraacetic acid calcium disodium salt (EDTA-Ca), calcium citrate, calcium acetate, calcium chloride, calcium nitrate, calcium maleate, calcium polyacrylate, calcium polymethacrylate and a combination thereof; a concentration of the calcium salt in the alkaline solution is 0.001˜10 mol/L.

In an embodiment, a molar ratio of the second phosphate to the calcium salt is (0.2˜5):1, preferably (0.8˜1.2):1.

In an embodiment, in step (S2), the alkaline solution is adjusted to pH 7-13 with sodium hydroxide, potassium hydroxide, aqueous ammonia or a combination thereof; and the soaking is performed at 5˜99° C. for 5 min˜48 h.

In an embodiment, the magnesium alloy substrate is Mg—Zn alloy, Mg—Ca alloy, Mg—Li alloy, Mg—Mn alloy or Mg—Re alloy.

Compared with the prior art, this application has the following beneficial effects.

(1) The existing coatings fail to enable the controllable degradation of the magnesium alloy, and most of the magnesium alloy implants are completely degraded within 6 months. However, the coating prepared in this application can extend the degradation time of the magnesium alloy implants to 12-18 months.

(2) Most of the existing coatings on magnesium alloy are single-layer coatings, whose thickness and uniformity are not easy to control, such that the degradation time is not controlled. The coating prepared herein has a double-layer structure consisting of a magnesium phosphate conversion inner layer and a hydroxyapatite outer layer, where the thickness of the hydroxyapatite coating can be controlled to adjust the degradation time.

(3) The existing coatings on magnesium alloy has insufficient bonding strength, and thus are prone to being scratched or falling off due to friction, especially when screwing in a bone screw. The coating prepared herein has a double-layer structure, where the magnesium phosphate conversion layer provides growth sites for the hydroxyapatite coating, which effectively improves the bonding strength between the coating and the substrate (more than 50 MPa).

(4) The preparation method provided herein has simple operation, mild reaction condition and low cost, and is applicable to magnesium and magnesium alloy implants of any complex shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a curve diagram showing weight loss of coated AZ31 magnesium alloys fabricated in Examples 1-2 and weight loss of an uncoated AZ31 magnesium alloy over time; and

FIG. 2 is a computerized tomography (CT) scanning image showing degradation of the uncoated Mg alloy screw and the Mg alloy screw coated with the coating fabricated in Example 1 after implantation.

DETAILED DESCRIPTION OF EMBODIMENTS

This application will be described in detail below with reference to the accompanying drawings and the embodiments. The following embodiments are merely illustrative, and are not intended to limit the disclosure. It should be noted that these embodiments and the features therein may be combined with each other in the case of no contradiction.

EXAMPLE 1

A coating, which was capable of controlling the degradation of magnesium alloys, was prepared on a surface of an AZ31 magnesium alloy bar through the following steps.

(1) The AZ31 magnesium alloy was processed into a sample with a size of 13×10 mm, which was sanded sequentially with 800#, 1200#, 2000# sandpapers, and subjected to ultrasonic cleaning in anhydrous acetone for 5 min to remove surface oil and blow drying.

(2) A solution containing magnesium nitrate (0.25 mol/L) and sodium dihydrogen phosphate (0.25 mol/L) was prepared, adjusted to pH 3.5 with dilute nitric acid, and heated to 75° C. in water bath.

(3) The AZ31 magnesium alloy sample obtained in step (1) was soaked into the solution prepared in step (2) for reaction for 60 min, rinsed with deionized water, and dried to obtain a magnesium phosphate conversion layer-coated AZ31 magnesium alloy sample.

(4) A solution containing calcium citrate (0.25 mol/L) and sodium dihydrogen phosphate (0.25 mol/L) was prepared and adjusted to pH to 9.5 with sodium hydroxide.

(5) The magnesium phosphate conversion layer-coated AZ31 magnesium alloy sample obtained in step (3) was soaked into the solution prepared in step (4) and heated to 95° C. in a water bath for reaction for 6 h. After the reaction, the sample was taken out, cooled, rinsed with deionized water and dried to obtain a sample with a uniform and complete surface coating.

(6) The bonding strength between the coating and the alloy substrate was measured to be 63 MPa by a universal mechanical testing machine.

EXAMPLE 2

A coating, which was capable of controlling the degradation of magnesium alloys, was prepared on a surface of an AZ31 magnesium alloy bar through the following steps.

(1) The AZ31 magnesium alloy was processed into a sample with a size of Φ3×10 mm, which was sanded sequentially with 800#, 1200#, 2000# sandpapers, and subjected to ultrasonic cleaning in anhydrous acetone for 5 min to remove surface oil and blow drying.

(2) A solution containing magnesium sulfate (0.5 mol/L) and potassium dihydrogen phosphate (0.5 mol/L) was prepared and adjusted to pH 2.5 with dilute sulfuric acid, and heated to 60° C. in water bath.

(3) The AZ31 magnesium alloy sample obtained in step (1) was soaked into the solution prepared in step (2) for reaction for 120 min, rinsed with deionized water, and dried to obtain a magnesium phosphate conversion layer-coated AZ31 magnesium alloy sample.

(4) A solution containing ethylenediaminetetraacetic acid calcium disodium salt (EDTA-Ca) (0.15 mol/L) and potassium dihydrogen phosphate (0.15 mol/L) was prepared and adjusted to pH to 9.0 with potassium hydroxide.

(5) The magnesium phosphate conversion layer-coated AZ31 magnesium alloy sample obtained in step (3) was soaked into the solution prepared in step (4) and heated to 80° C. in a water bath for reaction for 12 h. After the reaction, the sample was taken, cooled, and rinsed with deionized water and dried to obtain a AZ31 magnesium alloy sample with a surface evenly covered by a coating of white particles.

(6) The bonding strength between the coating and the substrate was measured to be 58 MPa by a universal mechanical testing machine.

EXAMPLE 3

A coating, which was capable of controlling the degradation of magnesium alloys, was prepared on a surface of a ZK60 magnesium alloy bar through the following steps.

(1) The ZK60 magnesium alloy was processed into a sample with a size of Φ3×10 mm, which was sanded sequentially with 800#, 1200#, 2000# sandpapers and subjected to ultrasonic cleaning in anhydrous acetone for 5 min to remove surface oil and blow drying.

(2) A solution containing magnesium dihydrogen phosphate (0.25 mol/L) was prepared, and adjusted to pH 3.0 with phosphoric acid, and heated to 70° C. in water bath.

(3) The ZK60 magnesium alloy sample obtained in step (1) was soaked into the solution prepared in step (2) for reaction for 45 min, rinsed with deionized water, and dried to obtain a magnesium phosphate conversion layer-coated ZK60 magnesium alloy sample.

(4) A solution containing calcium citrate (0.10 mol/L) and sodium dihydrogen phosphate (0.10 mol/L) was prepared and adjusted to pH to 9.0 with sodium hydroxide.

(5) The magnesium phosphate conversion layer-coated ZK60 magnesium alloy sample obtained in step (3) was soaked into the solution prepared in step (4) and heated to 80° C. in a water bath for reaction for 6 h. After the reaction, the sample was taken out, cooled, rinsed with deionized water and dried to obtain a ZK60 magnesium alloy sample with a uniform and complete surface coating.

(6) The bonding strength between the coating and the alloy substrate was measured to be 70 MPa by a universal mechanical testing machine.

EXAMPLE 4

A coating, which was capable of controlling the degradation of magnesium alloys, was prepared on a surface of an LZ91 magnesium alloy bar through the following steps.

(1) The LZ91 magnesium alloy was processed into a sample with a size of Φ3×10 mm, which was sanded sequentially with 800#, 1200#, 2000# sandpapers, and subjected to ultrasonic cleaning in anhydrous acetone for 5 min to remove surface oil and blow drying.

(2) A solution containing magnesium dihydrogen phosphate (0.35 mol/L) was prepared, adjusted to pH 3.0 with phosphoric acid, and heated to 75° C. in water bath.

(3) The LZ91 magnesium alloy sample obtained in step (1) was soaked into the solution prepared in step (2) for reaction for 60 min, rinsed with deionized water, and dried to obtain a magnesium phosphate conversion layer-coated LZ91 magnesium alloy.

(4) A solution containing ethylenediaminetetraacetic acid calcium disodium salt (EDTA-Ca) (0.15 mol/L) and sodium dihydrogen phosphate (0.15 mol/L) was prepared and adjusted to pH 7.5 with sodium hydroxide.

(5) The magnesium phosphate conversion layer-coated LZ91 magnesium alloy sample obtained in step (3) was soaked into the solution prepared in step (4) and heated to 80° C. in a water bath for reaction for 12 h. After the reaction, the sample was taken out, cooled, rinsed with deionized water and dried to obtain an LZ91 magnesium alloy sample with a uniform and complete surface coating.

(6) The bonding strength between the coating and the substrate was measured to be 73 MPa by a universal mechanical testing machine.

EXPERIMENTAL EXAMPLE 1

The samples prepared in the Examples 1 and 2 and uncoated AZ31 magnesium alloy sample were soaked into a solution containing 3% (w/w) sodium chloride at 37° C. to perform an accelerated degradation test. The uncoated AZ31 magnesium alloy sample was the same as the samples prepared in the Examples 1 and 2 in sizes. The solution was changed every 3-4 days, and all the samples were weighed every week. The surface corrosion and overall degradation of each sample were observed simultaneously to obtain a resulting weight loss curve shown in FIG. 1 . As shown in FIG. 1 , the weight of the uncoated magnesium alloy sample was greatly reduced by more than 50% after soaking for 30 days, while the weight of each of magnesium alloy samples prepared in Example 1 and 2 was only reduced by about 10% after soaking for 30 days, indicating that the coating greatly decelerated the degradation of the magnesium alloy.

EXPERIMENTAL EXAMPLE 2

The AZ31 magnesium alloy was processed into a bone screw shape. The bone nail made of coated AZ31 magnesium alloy was prepared according to the method in Example 1, and the bone nail made of uncoated AZ31 magnesium alloy was taken as a control example. The bone nail made of coated AZ31 magnesium alloy and the bone nail made of uncoated AZ31 magnesium alloy were respectively implanted into a tibial plateau of the left leg and a tibial plateau of a right leg s of a goat, and then scanned by computerized tomography (CT) scanning at 3, 6, 12, and 18 months to observe the degradation of magnesium alloy bone nails. As shown in FIG. 2 , the bone nail made of uncoated magnesium alloy began to degrade and generate a large amount of gas after implantation, such that cavities were formed in the bone tissue (marked by white arrow in FIG. 2 ). 6 months after implantation, the bone nail made of uncoated magnesium alloy was basically degraded. 12 and 18 months after implantation, the bone nail made of uncoated magnesium alloy could not be seen. However, 12 months after the implantation, the bone nail made of coated magnesium alloy provided herein still could be clearly seen, and the thread structure of the bone nail almost remains unchanged. Referring to FIG. 2 , 18 months after implantation, the bone nail made of coated magnesium alloy almost disappeared. Moreover, the bone nail made of coated magnesium alloy provided herein was rarely degraded within 12 months, and the degradation was completed within 12 to 18 months. It can be concluded that the surface coating for the magnesium alloy provided herein effectively controlled the degradation of magnesium alloy in vivo, and the degradation time is controlled within 12 to 18 months.

Described above are merely preferred embodiments of this application, which are not intended to limit this application. It should be understood that various modifications, replacements and changes made by hose skilled in the art without departing from the spirit of the application should still fall within the scope of the present application defined by the appended claims. 

What is claimed is:
 1. A surface coating for degradable magnesium and magnesium alloy, comprising: an inner layer; and an outer layer; wherein the inner layer is a magnesium phosphate conversion layer, and the outer layer is a hydroxyapatite layer.
 2. The surface coating of claim 1, wherein the magnesium phosphate conversion layer has a thickness of 50 nm-10 μm; and the hydroxyapatite layer has a thickness of 0.1 μm-500 μm.
 3. The surface coating of claim 1, wherein a bonding strength between the surface coating and a magnesium substrate or a magnesium alloy substrate is equal to or larger than 50 MPa; and an atomic ratio of calcium to phosphorus in the hydroxyapatite layer is (1.5-1.67):1.
 4. A method for preparing the surface coating of claim 1, comprising: (S1) soaking a magnesium or magnesium alloy substrate in an acidic solution containing a magnesium salt and a first phosphate under heating to form a magnesium phosphate conversion layer on a surface of the magnesium or magnesium alloy substrate; and (S2) transferring the magnesium or magnesium alloy substrate with the magnesium phosphate conversion layer to an alkaline solution containing a calcium salt and a second phosphate followed by soaking under heating to form a hydroxyapatite layer on a surface of the magnesium phosphate conversion layer.
 5. The method of claim 4, wherein in step (S1), the magnesium salt is selected from the group consisting of magnesium nitrate, magnesium chloride, magnesium sulfate, magnesium phosphate, magnesium perchlorate, magnesium acetate, magnesium hydroxide and a combination thereof; a concentration of the magnesium salt in the acidic solution is 0.001˜10 mol/L; the first phosphate is selected from the group consisting of sodium phosphate, potassium phosphate, magnesium phosphate, ammonium phosphate, calcium phosphate, and a combination thereof; and a concentration of the first phosphate in the acidic solution is 0.001˜10 mol/L.
 6. The method of claim 5, wherein a molar ratio of magnesium ions to phosphate ions in the acidic solution in step (S1) is (0.2˜5):1.
 7. The method of claim 4, wherein in step (S1), the acidic solution is adjusted to pH 2˜7 with nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid or a combination thereof; and the soaking is performed at 5˜99° C. for 5 min˜12 h.
 8. The method of claim 4, wherein in step (S2), the second phosphate is selected from the group consisting of sodium phosphate, potassium phosphate, magnesium phosphate, ammonium phosphate, calcium phosphate, and a combination thereof; and a concentration of the second phosphate in the alkaline solution is 0.001˜10 mol/L; the calcium salt is selected from the group consisting of calcium phosphate, ethylenediaminetetraacetic acid calcium disodium salt (EDTA-Ca), calcium citrate, calcium acetate, calcium chloride, calcium nitrate, calcium maleate, calcium polyacrylate, calcium polymethacrylate and a combination thereof; a concentration of the calcium salt in the alkaline solution is 0.001˜10 mol/L; and a molar ratio of the second phosphate to the calcium salt is (0.2˜5):1.
 9. The method of claim 4, wherein in step (S2), the alkaline solution is adjusted to pH 7˜13 with sodium hydroxide, potassium hydroxide, aqueous ammonia or a combination thereof; and the soaking is performed at 5˜99° C. for 5 min˜48 h.
 10. The method of claim 4, wherein the magnesium alloy substrate is Mg—Zn alloy, Mg—Ca alloy, Mg—Li alloy, Mg—Mn alloy or Mg—Re alloy. 